A liquid crystal display device having a liquid crystal display panel in which a liquid crystal is sealed between a pair of substrates is used as a display portion of televisions, displays for personal computers, and portable electronic equipment such as mobile telephone terminals. In this case, one of a pair of substrates (hereinafter referred to as a TFT substrate) is formed with scanning lines, signal lines, thin-film transistors, pixel electrodes and alignment layers on the surface of an insulating substrate such as a glass substrate. Further, the other of the pair of substrates (hereinafter referred to as a counter substrate or a color filter substrate) is formed with a light cut off layer (hereinafter referred to as a black matrix layer), color filters and alignment layers, etc.
The TFT substrate and the counter substrate used for existent liquid crystal display panels are manufactured, for example, by using a glass substrate having thickness of about 0.5 mm to 1.1 mm as the insulating substrate.
By the way, for the liquid crystal display panel, decrease in the thickness of the insulating substrate has been investigated in order to decrease the weight of the liquid crystal display device (liquid crystal display module). However, in a case of using a glass substrate as the insulating substrate, when the thickness is 0.5 mm or less, it involves a problem of tending to be cracked, for example, in the course of manufacture or during use of a completed liquid crystal display device. As one of countermeasures for the problems, a method of forming the insulating substrate for the TFT substrate and the counter substrate, for example, with a plastic substrate (resin substrate) has been proposed.
A liquid crystal display panel prepared by using a plastic substrate for the insulating substrate of the TFT substrate and the counter substrate has an advantage that it is light in the weight and less cracked and, in addition, it may be considered an advantage for example, that the display surface can be formed in a curved surface taking the advantage of the flexibility of the plastic substrate, thereby increasing the degree of freedom in view of design. Further, it is considered that such a liquid crystal display panel can enhance the possibility of obtaining a rollable liquid crystal display device, for example, of making the size smaller, for example, by bending or rolling up the device when it is not used.
However, while the heat resistance of glass substrates is about 600° C., the heat resistance for plastic substrate is about 200° C. In the thin-film transistor for usual liquid crystal panels, amorphous silicon (a-Si) or polysilicon is used as a semiconductor layer and a step of forming the thin-film transistor has a step of applying a treatment at a high temperature of about 300° C. or higher. Therefore, manufacture of the TFT substrate by using a plastic substrate requires a countermeasure, for example, of improving the heat resistance of the plastic substrate or lowering the temperature when the thin-film transistor is formed.
When plastic materials (resin materials) are classified in view of the heat resistance, it has been known that the heat resistance, for example, of a polyimide type resin used for the insulating substrate of a flexible wiring circuit board is relatively high. However, since usual polyimide type resins are colored yellow to brown, it is difficult to use them as they are for the insulating substrate of the liquid crystal display panel. By the way, it has been found in recent years that the transparency of the polyimide type resin is improved by modifying an aromatic moiety in the polyimide structure to an alicyclic structure (for example, refer to JP-A No. 2006-199945). Then, it can be said that this enhances the possibility of attaining the liquid crystal display panel using the plastic substrate.
However, the polyimide type resin has retardation inherent to the structure and highly transparent polyimide type resin disclosed in JP-A No. 2006-199945 also has retardation. Accordingly, the polyimide type resin is not suitable as the material for the insulating substrate of the liquid crystal display panel used by bonding a polarizer such as a polarization plate or a polarization film.
Further, also in view of the coefficient of thermal expansion, while the coefficient of thermal expansion of a glass substrate is several ppm/K, the coefficient of thermal expansion of a plastic substrate is from several tens ppm/K to one hundred and several tens ppm/K. Accordingly, in a case of treatment at a high temperature such as in the step of forming a thin-film transistor, the plastic substrate exhibits more elongation (thermal expansion) compared with the glass substrate. This results in fluctuation of the size or positional deviation for formation and it is expected, for example, that scattering of properties of thin-film transistors increases between each of the pixels to deteriorate the display property of the liquid crystal display device.
Furthermore, different from the glass substrate, the plastic substrate is soft and liable to be deformed. Accordingly, the TFT substrate and the counter substrate using the plastic substrate involves a problem that it is extremely difficult to manufacture them in the existent manufacturing apparatus for the TFT substrate and the counter substrate using the glass substrate. For such a problem, it may be considered a method of changing the manufacturing apparatus for the TFT substrate and the counter substrate, for example, to roll-to-roll system which is used when flexible wiring boards, etc. are manufactured.
On the other hand, as a method of preparing a TFT substrate and a counter substrate using a plastic substrate by utilizing an apparatus for manufacturing the TFT substrate and the counter substrate using the glass substrate, a method, for example, of transferring a thin-film transistor or the like formed on the glass substrate to a plastic substrate has been investigated. The method of transferring the thin-film transistor or the like includes, for example, a method of etching a glass substrate formed with a thin-film transistor or the like thereby decreasing the thickness or a method of previously forming a separation layer to a glass substrate and separating the separation layer after forming the thin-film transistor or the like. In the transfer methods described above, the thickness of the glass substrate formed with the thin-film transistors, etc. is decreased and they are relocated to the plastic substrate. Further, in another transfer method, a plastic substrate is bonded on a glass substrate and, after forming the thin-film transistors, etc. on the plastic substrate, the glass substrate is separated (for example, refer to JP-A No. 2002-033464).
By the way, in existent liquid crystal display devices, a polarizer plate or polarizer is bonded to the surface of the liquid crystal display panel directed to the outer side. On the other contrary, for the liquid crystal display device in recent years, a method of incorporating the polarizer in the TFT substrate and the counter substrate and disposing the polarizer between the insulating substrate of the TFT substrate and the insulating substrate of the counter substrate is proposed, for example, in JP-A No. 11-084413.
Further, for the polarizer in the existent liquid crystal display devices, polarizers formed by dying polyvinyl alcohol with iodine and stretching the same upon preparation into a film thereby arranging iodine molecules in the stretching direction are sandwiched between protective films made of triacetyl cellulose are used. On the contrary, in the liquid crystal display device in recent years, a method of using a polarizer referred to as a wire grid polarizer formed by utilizing fine fabrication technique has been proposed, for example, in JP-A No. 2007-102174.